Hydraulic couplers for quickly connecting and disconnecting construction attachments from excavating equipment are well known. A major problem with existing couplers is that a failure of the primary locking force, typically a hydraulic failure, can cause the attachment to swing or even completely detach from the coupler posing a safety hazard. Another problem is operator misuse or accidental operation of the coupler again leading to a safety hazard. Safety devices are fitted to couplers which prevent the attachment separating from the coupler in the event of a failure of the coupler or its associated locking components. However in all couplers there is little or no indication of a failure having taken place. Most of the couplers using a device to power the lock on the front pin receiving aperture use either a direct connection between the main actuator, which is normally a hydraulic cylinder, and the front lock or a separate hydraulic cylinder to actuate the front lock. Some couplers also use a linkage from the rear hook to actuate the front lock. The problem with all of these designs is that the front hook will retract when the cylinder is actuated. The use of a gravity pendulum device was disclosed in European Patent Application EP 2161378, which meant that the front lock would only be released in a certain coupler orientation. However for this pendulum device to work effectively it required the use of two hydraulic cylinders. An additional small bore hydraulic cylinder is fitted in the coupler to operate the front lock.